Desiccation product removal means



Nov. 20, 195] Filed March 23, 1950 r R. E. COLWELL DESICCATION PRODUCTREMOVAL MEANS 2 SHEETS SHEET 1 [1 I J x J I f /6 /0 INVENTOR.

Roan? T E. COL WELL Nov. 20, 1951 R. E. COLWELL 2,575,748

DESICCATION PRODUCT REMOVAL MEANS Filed March 23, 1950 2 SHEETSSHEET 2mmvrok. F055,? 7 f. 004 h/fLL BY 4, ATTORNEY Patented Nov. 20, 1951DESICCATION PRODUCT REMOVAL DIEANS Robert E. Colwell, Springfield, Mam,assignor to Monsanto Chemical Company, St. Louis, Mo., a

corporation of Delaware Application March 23, 1950, Serial No. 151,384

Claims. 1

This invention relates to spray drying processes and apparatus therefor.More particularly, the invention relates to an improved method and meansfor controlling the flow of the gaseous drying medium having dried solidmaterial suspended therein.

Spray dryers are used as rapid and efllcient means for removing volatilematerials from various types of solids. Briefly, they operate asfollows: (1) the solid material dissolved or suspended in a. volatilematerial is introduced into a tower in the form of a spray; (2) a hotgaseous medium is introduced into the tower and is directed so that itmixes quickly and thoroughly with the spray; (3) the volatile materialevaporates and becomes greatly diluted by the gaseous mediumprecipitating the solid material in the form of a dry powder; (4) thegaseous medium is continuously removed from the drying chamber carryingwith it the major portion of the dry powder; (5) a portion of the drypowder collects on the bottom of the tower and must be removedtherefrom.

Many of the problems concerned with the introduction of the material tobe dried and the drying medium have been solved, but the removal of thegaseous mixture and the dried powder from the spray drying chamber stillpresents difflculties. For example, the gaseous drying medium tends tochannel, 1. e., to travel directly to and through the exit port therebyminimizing the amount of drying action. In addition, a substantialamount of dried material collects on the bottom of the dryer and must beremoved therefrom before it re-absorbs the solvent or suspending agentand becomes tacky. Still another difllculty particularly pertinent toresinous materials is the problem of cooling the dried material beforeit enters the collecting devices so that it will not agglomerate to formlarge lumps or adhere to the surfaces of the apparatus.

Various devices have been suggested for solving the problems presentedby the removal of the gaseous medium and the dried powders. Thesedevices have ranged from the use of jets to introduce the drying mediumwith a swirling motion and with sufllcient force to project the dryingmedium against the walls, to the use of mechanical and air sweeps tochange the direction of flow of the drying medium through the lower partof the chamber and to remove dried material from the bottom of thetower.

Many of the remedies suggested have resulted in a partial solution ofthe problems but at the same time have created new problems. Forexample, the mechanic sweep is fairly eflicient for removing driedpowder from the floor of the chamber and reduces the channeling of thedrying medium to some extent. However, the various parts of the sweepshave served as additional surfaces on which the dried powder accumulatednecessitating frequent shutdowns for cleaning operations.

An object of this invention is to provide an improved spray dryingapparatus.

A further object is to provide a means for preventing channeling of thegaseous drying medium.

Another object is to provide a means for cooling the dried material andthe drying medium near the bottom of the chamber.

Still another object is to provide a means for expediting the removal ofthe dried material from the drying chamber.

Other objects will be evident from an inspection of the accompanyingdrawings in which:

Figure 1 is a diagrammatic view. partly in section, of a spray dryingtower; and

Figures 2, 3 and 4 are diagrammatic sectional views of three embodimentsof the invention.

These and other objects are attained by inserting a truncated conicalbaflle into the chamber of the dryer concentrically over the exit portthereof and by providing a means for delivering a relatively coolgaseous medium into the dryin chamber through the top of the cone.

Referring to the figures, spray tower l is provided at the top with aconduit 5 leading into the drying chamber 6 and carrying atomizing meansI. Also, at the top of the tower and sur-' rounding conduit 5, there isprovided conduit-l leading to port 9. Floor Ill is provided with an exitport I I.

A truncated conical baflle 12, having base I. is located concentricallyover exit port H and above floor III. A conduit l3 leads from oumide ofthe spray chamber into the base of conical battle 12. At the top of saidbaiile is a port It. Situated directly over port It and spaced a short;distance therefrom is a secondary baiiie ll.

Figure 1 may be used to describe in detail the normal operation of thedryer. A solution of asolid in a volatile liquid or a fluid mixture of asolid with a volatile liquid such as an aqueous slurry is introducedthrough conduit 5 into chemher 6 where atomizer 1 converts the liquid tothe formof a fine spray. A heated gas such as air. nitrogen, carbondioxide, etc. is introduced into the chamber through conduit 8 and port9. The temperature of the gas is sui'ilciently high to be readilyre-absorbed by the dried solid. The

dilute mixture of volatilized liquid and gaseous heating medium retainsa major proportion of the pulverulent dried solids and carries it downthrough the tower. As the gaseous stream flows down through the tower,the central portion of it impinges on the sides of the baiiie i2 and isdeflected towards the walls of the chamber as shown by the solid arrows.current of a relatively cool gaseous medium simultaneously fed throughconduit i3, and out through port i4, against the under side of secondarybame it, whence it flows down the sides of baiile 52 as shown by thebroken arrows. The suspension of solid particles in the gaseous mediumeventually passes under the base it of bailie it and out through port iito a collecting device not shown.

In the spray drying operation, the flow of the gaseous media and of thedried particles suspended therein is controlled by maintaining apressure diiferential between the inlet and exit ports of the dryingtower, such that the lower pressure is always at the exit port. It isimmaterial whether the pressure diiferential is the difference betweentwo positive pressures, between a positive and a negative pressure,between atmospheric pressure and a negative pressure, or between twonegative pressures. A standard method of providing a pressure difier-'ential is to insert an exhaust fan in the device for receiving thedried material. If the fan is of suilicient size and proper design, itmay be used as the sole source of pressure differential. However, it isalso feasible to provide a blower to force the desiccating gas into thedrying chamber -under positive pressure.

The essence of this invention is the use of the conical 'bailie and thepassage of a relatively cool gaseous medium through and over the baffle.

The size and shape of the conical bafile i2 is critical in severalrespects. The base of the cone 24 must extend beyond the periphery ofexit port ii to prevent channeling of the gaseous media, and it mustalso be located far enough above the floor of the drying chamber toprevent excessive pressure drop between the chamber and the exit port.In general, the area of the base of the cone 2t should be from 50% to200% greater than the area of the exit port. At smaller diameters,channeling of the gaseous media is not prevented and at largerdiameters, substantial benefits are not obtained.

The shape of heme it may be varied and is described above as conical" inthe sense set forth in Webster's New International Dictionary publishedby G. 8: C. Merriam (30., i. e., a solid bounded by a circular or otherclosed plane base and the surface formed by line segments joining everypoint of the border of the base to a common point called the vertex.Thus, the baflle instead of having a round base and round sides may havepolygonal base and sides corresponding thereto.

The height of the conical baille i2 may be varied within certain limits.It should be at least equal to the radius of the base of the cone andmaybe increased to about 4 times said radius. In diiferent language, theangle it between the base of the cone and the side thereof may be variedbetween 45 and 75. If the angle is less than 45, the slope of the sidesis so gentle that dried material may collect thereon in spite of theflow of gaseous medium. If the angle is greater than 75, the sides ofthe balls are too steep to eil'ectively deflect the desiccating mediumand channeling occurs...

The baflie I2 may be solid except for the conduit 13 passingtherethrough or it may be hollow with conduit l3 extending only to thebase thereof as shown in Figures 1 and 2. In practice, it is preferableto use a hollow baiiie since it permits the relatively cool gaseousmedium entering from;

conduit I8 to-maintaln the walls of the baille at a temperature lowenough to prevent adhesion of the dried particles thereto.

Baflie i2 may be held in place by conduit l3 or it may be mounted onsuitable brackets or standards, not shown.

The temperature of the relatively cool gaseous medium'introduced throughconduit 13 ,may be varied over a wide ransewith the restriction that itshould be substantially cooler than the desiccating medium andpreferably should be at a temperature below the softening point of thedried material. For economical reasons, the temperature of therelatively cool gas will be atmospheric temperature although it may becooled substantially below that point or heated above it when necessary.

Figures 2-, 3 and 4 are diagrammatic representations of specificembodiments of this invention.

In Figure 2, conical baille I2 is hollow and has a base 24 which isparallel to the floor of the drying chamber. Conduit l3 terminates atthe base 24. Located above baiile I2 is a smaller bafile i5 mounted onbrackets 2|. The small baffle serves to deflect the cool gaseous mediumissuing from port It down the sides of baffle I2. Baille i5 ispreferably of the same shape as baflle i2 but it may be of any generallyconical shape. The periphery of the base ofbafile I5 extends beyond andabove the periphery of port H.

In Figure 3, the base 16 of baiiie l2 slants away from the floor of thedrying chamber from the periphery of the loathe to the periphery ofconduit. it. The slanting base serves to eliminate possible excesspressure differentials which may sometimes interfere with the smoothflow of the chamber contents through port H.

The preferred embodiment of this invention is shown in Figure 1, whereinthe base of baffle i2 is slanted from the periphery 22 of the bafiie toa point 23 opposite the periphery of port II. The remainder of the baseiii of baflle I2 is parallel to the base of the drying chamber. In thismodification, the slant of the base It from points 22 to 23 is critical.The slope of the baffle should be constructed according to the formulaRH=rh wherein R is a radius from the center of the base it of baflle i2drawn to the periphery thereof, H is the height of base l6 above floorill at point 22, r is any radius drawn from the center of the base toany point between 22 and 23 and h is the height of the base above floorH) at the outer extremity of 1'. By sloping the base It of bafiie i2according to this formula, linear flow of the gaseous medium with agradual pressure drop is attained. As a result of this, all theparticles suspended in the gaseous media are swept into exit port II andto the collecting device.

Figure 4 depicts a modification of the preferred embodiment of thisinvention, wherein conduit it passes completely through bafiie i2 andterminates in port it.

In a further embodiment of this invention, the floor of the dryingchamber is separated from the walls thereof by circumferential openingor gap l8. Means such as a bustle pipe 20 surrounds said gap It] andserves to introduce a relatively cool gaseous medium across the floor IDof the chamber. The gaseous medium coming through gap i9 sweeps anydried material which has accumulated on the floor In of the dryertowards the periphery of bafile it where it is picked up by the gaseousmedium flowing to the exit port and is carried under the baiiie with themedium. By the addition of this secondary means for introducing a streamof relatively cool gaseous medium at the periphery of the floor of thedrying chamber, no accumulation of dried material occurs even at theextreme outer edge of the dryer floor. This secondary means forintroducing cool media at the bottom of the dryer is the subject ofco-pending application Serial No. 50,355, filed September 21, 1948, andnow matured into Patent Number 2,566,292, in the name of Frank E. ReeseuBy using the apparatus described, it has been found possible tospray-dry a wide variety of materials having varied softening points,with the elimination of substantial accumulations of dried material atany point in the drying chamber. Furthermore, the dried material isdelivered to the collecting devices in a relatively cool state so thatno caking or lumping occurs in such collecting devices.

It is obvious that variations may be made in the processes and apparatusof this invention without departing from the spirit and scope thereof asdefined in the appended claims.

What is claimed is:

l. in a spray drying chamber, means at the bottom of the chambercomprising a centrally located exit port for removing a gaseous mediumcontaining solid particles suspended therein, a baille in the shape of atruncated cone located concentrically over the exit port, the diameterof the base of said cone being substantially greater than the diameterof the exit port, the area of the base of said cone being from 50% to206% greater than the area of the exit port and the height of said conebeing from 1 to 4 times the radius of the base of the cone, means for introducing a gaseous medium into the spray drying chamber through the topof said cone, and means for deflecting said gaseous medium downward overthe outer surface of said cone.

Ill

2. A spray drying apparatus as in claim 1 wherein the base of thetruncated cone is hat and parallel to the floor of the drying chain her.

3. In a spray drying chamber means adjacent the bottom of said chambercomprising a centrally located exit port for removing a gaseous mediumcontaining solid particles suspended therein, a truncated cone locatedconcentrically over said exit port, the diameter of the base of saidcone being substantially greater than the diameter of the exit port, andthe surface of the base of the cone receding from the floor of the dryeraccording to the relation RH =rh wherein R is the radius of the base ofthe cone at the periphery thereof and H is the distance between thefloor of the chamber and the periphery of the cone, r is any otherradius from the center of the base of the cone to any point on the basenot on the periphery thereoi and h is the distance between the base ofthe cone and the floor of the chamber at the outer extremity of r. aconduit leading through the exit port into the base of said cone, anopening at the top of said cone, means for forcing a relatively coolgaseous medium through said conduit and said cone into the said chamberand a bailie located adjacent the top of said cone serving to deflectthe gaseous medium downward over the outer surface of said cone.

4. A spray drying apparatus as in claim 3 in which the recession of thebase of the truncated cone is terminated at points opposite theperiphery of the exit port.

5. A spray drying apparatus as in claim 4 in which the conduit leadingthrough the exit port into the base of the truncated cone continuesthrough said cone and terminates at the opening at the top of said cone.

ROBERT E. COLWELL.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,946,566 Bowen Feb. 13, 19342,449,366 Bowen Sept. 14, 1948

